1. Field of the Invention
The present invention relates to an optical pickup apparatus for performing at least one of information recording, information reproduction, and information erasing with respect to an optical recording medium such for example as a double-layered optical disc, as well as to a drive apparatus having the same.
2. Description of the Related Art
As optical recording media subjected to at least one of information recording, information reproduction, and information erasing, optical discs such as a CD (Compact Disc), a DVD (Digital Versatile Disc), and a MD (Mini Disc) have been frequently used. Such an optical recording medium has a track portion formed on its information recording layer. In order to perform at least one of information recording, information reproduction, and information erasing, it is necessary to cause a light spot convergently illuminated on the information recording layer to follow the track portion of the optical recording medium rotating at a high speed.
In order to cause the light spot to follow the track portion of the optical recording medium; that is, to exercise tracking control, a method for detecting light quantity differences among a plurality of light-receiving sections has been used, such as a push-pull method (hereafter referred to as “the PP method”) and a differential push-pull method (hereafter referred to as “the DPP method”).
According to the PP method, light reflected from the track portion of the optical recording medium is divided into a plurality of light beams and received by a plurality of light-receiving sections arranged symmetrically in respect to the center of the track portion. On the basis of the difference in output among the plurality of light-receiving sections, the amount of detrack is detected. The detrack amount is detected as a push-pull signal (hereafter referred to as “a PP signal”) which is one of tracking error signals and serves as a signal indicative of a position relative to the track portion. When there is no output difference as described above, it is judged that a just track condition is attained.
For the purpose of achieving miniaturization, a low-profile structure, and high reliability as to an optical pickup apparatus of conventional design that adopts the PP method, there is disclosed a technique for utilizing a hologram. Hereinafter, a conventional optical pickup apparatus which adopts the PP method will be described. FIG. 9 is a configuration diagram showing the conventional optical pickup apparatus 800 in a simplified manner. FIG. 10 is an enlarged perspective view showing a part of a light integration unit provided in the conventional optical pickup apparatus 800 shown in FIG. 9.
The conventional optical pickup apparatus 800 shown in FIG. 9 includes a light integration unit including a hologram 801 and an optical system in which light emitted from the light integration unit is condensed on an optical recording medium and reflection light from the optical recording medium is directed to the light integration unit. In the optical pickup apparatus 800, outgoing light from a semiconductor laser chip 802 is diffracted by the hologram 801. Out of the diffraction light beams, a zero-order diffraction light beam passes through a coupling lens 803, a quarter (hereafter referred to as “¼”) wavelength plate 804, an aperture stop 805, and an objective lens 806, and is eventually illuminated convergently on an information recording layer 808 of the optical recording medium such as a double-layered optical disc 807. Then, a return light therefrom is directed to the hologram 801 through the objective lens 806, the aperture stop 805, the ¼ wavelength plate 804, and the coupling lens 803.
Herein, an X axis, a Y axis, and a Z axis in a three-dimensional orthogonal coordinate system shown in FIG. 9 are defined as follows. The Z axis equates to an axis extending in a direction axially of the light having been emitted from the semiconductor laser chip 802 and condensed on the double-layered optical disc 807. The X axis equates to an axis located in a direction in which a line segment connecting the center of the double-layered optical disc 807 and the light condensing position extends within a virtual plane perpendicular to the Z axis. The X axis is coincident with a direction radially of the double-layered optical disc 807. Hereafter, the direction in which the X axis extends is referred to as “the radial (X) direction”. The Y axis equates to an axis extending in a direction perpendicular to the X axis within the virtual plane perpendicular to the Z axis. The Y axis is coincident with the direction of the tangent to a track formed in the double-layered optical disc 807. Hereafter, the direction in which the Y axis extends is referred to as “the track (Y) direction”. The definitions of those three axes are used in common throughout the present description.
As shown in FIG. 10, the hologram 801 is divided into three segmented regions: 801a, 801b, and 801c by a division line L6 extending in the radial (X) direction of the optical disc and a division line L7 extending from the center of the division line L6 in a direction corresponding to the track (Y) direction of the optical disc. A diffraction light beam derived by the segmented region 801a of the hologram 801 is condensed on a division line L8 for dividing a light-receiving element into a light-receiving element 809a and a light-receiving element 809b. A diffraction light beam derived by the segmented region 801b and a diffraction light beam derived by the segmented region 801c are condensed on a light-receiving element 809c and a light-receiving element 809d, respectively. Provided that output signals from the light-receiving elements 809a, 809b, 809c, and 809d are indicated by H1, H2, H3, and H4, respectively, then a focus error signal is obtained by computation based on a formula: (H1−H2) in accordance with a single knife-edge method. Moreover, a PP signal known as one of tracking error signals is obtained by computation based on a formula: (H3−H4) in accordance with the PP method. Further, an information signal is obtained by computation based on a formula: (H1+H2+H3+H4).
In the optical pickup apparatus employing such a hologram 801 that is disclosed in, for example, Japanese Unexamined Patent Publication JP-A 9-161282 (1997), a light-receiving element adapted for focus error signal correction is additionally provided in the structure thus far described. This makes it possible to obtain an offset-free focus error signal even in a case of using a DVD in which a distance between adjacent recording/reproduction layers is small.
However, the PP method poses the following problem. When the objective lens is shifted, the position of the return light at the light-receiving section is caused to vary, inconsequence whereof there results an offset in the tracking error signal even in the absence of detrack.
On the other hand, according to the DPP method, by a diffraction grating disposed between a light source for emitting light and an optical recording medium, the light emitted from the light source is branched into one main beam and two sub beams. The beams are illuminated on the optical recording medium. With respect to each of the main beam and the sub beams, tracking control is exercised in the manner as described previously to detect a PP signal. In this way, since tracking control is exercised by using not only the main beam but also the two sub beams, it is possible to suppress an offset which occurs in the case of adopting the PP method. However, the DPP method also poses the following problem. Since the three beams are generated from a single light emitted from the light source, the main beam to be used in information recording or information reproduction becomes smaller in light quantity relative to the light emitted from the light source. This leads to a decrease in light use efficiency. As a result, the speed of information recording or the speed of information reproduction becomes slower, which causes hindrance to the accomplishment of high-speed recording and reproduction.
In this regard, there have been proposed various methods that are based on the PP method known as a one-beam method and nevertheless allow correction of an offset which occurs in the PP method by utilizing, for example, an objective lens shift signal corresponding to the shifting of an objective lens. For example, Japanese Unexamined Patent Publication JP-A 8-306057 (1996) discloses an optical head that succeeds in reducing an offset. In this construction, a beam of reflection light from an information recording medium is received by a 6-split detector, and computation is performed on a light detection signal in each light-receiving region so as to cancel out the shifting of the reflection light beam in keeping with the shifting of an objective lens in a tracking direction. In this way, an offset occurring in a tracking error signal in accompaniment with the shifting of the objective lens can be reduced.
In recent years, as an optical recording medium, a BD (Blu-ray Disc) has been in wide use in which a light transmitting layer on which is formed an information recording layer has a thickness of 0.1 mm. In this BD, the distance between the information recording layer and the surface of the light transmitting layer (hereafter also referred to as “light transmitting layer thickness”) is set to be as small as 0.075 mm to 0.100 mm. Therefore, as compared with a case of using a DVD or the like medium having a relatively large light transmitting layer thickness, the influence of stray light stemming from reflection light from the surface of the light transmitting layer is so great that there is a possibility that focus control and tracking control cannot be conducted with high accuracy.
Neither the conventional pickup apparatus disclosed in JP-A 9-161282 (1997) nor that disclosed in JP-A 8-306057 (1996) is able to solve the above-described problem caused by the use of a BD.